More than 100 MPs are now part of a WhatsApp group run by former Brexit Minister Steve Baker, which will be used to coordinate rebellions against the Government, according to the Telegraph.

HuffPost UK understands former Brexit Secretary David Davis is planning to make his first Parliamentary contribution since quitting Cabinet last Sunday in the debate on the Bill, most likely on Monday.

But most attention is likely to be on the former Foreign Secretary Boris Johnson, who could use a resignation statement to the Commons to launch a direct challenge to May’s leadership.

Such a move would be reminiscent of the events which saw Margaret Thatcher booted out of Downing Street by her MPs in 1990, when outgoing Deputy Prime Minister Geoffrey Howe delivered a damning speech from the backbenches.

Thatcher was out of office within three weeks.

Theresa May launched a defence of her Brexit plan on Sunday, but saw further resignations from Government.

Speaking on the BBC’s Andrew Marr show on Sunday, May refused to explicitly say whether she would contest a leadership contest, instead claiming she was “in this for the long term.”

Mansfield MP Ben Bradley, who quit as a Tory vice-chairman last week in protest at the Brexit plans, urged the PM to change her stance.

He told HuffPost UK: “It’s important that we all try to lay out the alternative Brexit vision this week, the big guns like Boris and DD have a big role to play in doing that, and we have to try to change the policy.

“If the PM is willing to listen and we can find a direction that delivers what was promised, then we won’t have to have all these conversations about leadership.”

But it is not just May who is under pressure.

Johnson, who helped lead the successful Vote Leave campaign in the referendum, is facing calls from both sides of the Brexit debate to finally set out his vision for the negotiations with Brussels.

One Remain-backing former minister had a simple message for Johnson.

“Put up,” they told HuffPost UK.

One Tory Brexiteer told HuffPost UK they were worried Johnson’s intervention could be “nuclear”, while another said the former Mayor of London had backed himself into a corner.

The senior Tory source said: “If he stands up and tries to calm it all down, people will ask why did you resign? Or if he doubles down and does a Geoffrey Howe, he can hardly then say to the PM she’s got his full support.”

The MP said the only way Johnson could not cause an incident is by “making that statement so bland and saying ‘I hope I’m wrong’.”

Geoffrey Howe's resignation statement in 1990 sparked the end of Margaret Thatcher's premiership

If Johnson does issue a call to arms and present an alternative vision for the Brexit negotiations, it could prompt more resignations from Cabinet.

The Sunday Times has reported that Work and Pensions Secretary Esther McVey and International Development Penny Mordaunt are both considering with to quit in protest at May’s Brexit plan.

In order to trigger a vote of no confidence in May, 48 Tory MPs need to send in letters to the chair of the Conservative backbench committee, Sir Graham Brady.

It is not known how many letters have already been submitted, but last week North West Leicestershire MP Andrew Bridgen went public with his call for May to go and it is rumoured the number is already in the 40s.

Even if a vote of no confidence is triggered, those wishing to oust May would need to get a majority of MPs to back getting rid of the PM – although if a sizeable minority want her gone she may fall on her sword.

Under the party rules, May would be unable to stand in the subsequent leadership contest – meaning a new Prime Minister could be in place by the party conference in October.

The MP for Witney – who replaced former Prime Minister David Cameron in the constituency – tweeted his resignation on Sunday afternoon, saying: “I have taken very difficult decision to resign position as PPS to express discontent with #Chequers in votes tomorrow. I had to think who I wanted to see in the mirror for the rest of my life. I cannot tell the people of WOxon that I support the proposals in their current form.”

I have taken very difficult decision to resign position as PPS to express discontent with #Chequers in votes tomorrow. I had to think who I wanted to see in the mirror for the rest of my life. I cannot tell the people of WOxon that I support the proposals in their current form.